Chromium plating from an organic/aqueous medium

ABSTRACT

DEPOSITION OF BRIGHT, DECORATIVE CHROMIUM PLATE IS OBTAINED FROM A CHROMIUM PLATING MEDIUM THAT IS A HOMOGENEOUS LIQUID BLEND OF WATER WITH DIPOLAR ORGANIC COMPOUND. THE MEDIUM CONTAINS A COMPLEX, WATER-SOLUBLE TRIVALENT CHROMIC COMPOUND FOR PLATING THAT CONTAINS CARBOXYLIC ACID CONSTITUENTS AND HALOGEN CONSTITUENTS AND EXHIBITS READY WATER SOLUBILITY. THE CHROMIUM DEPOSITS FROM SUCH A MEDIUM EXHIBIT ENHANCED PLATING COVERAGE.

United States Patent 3,706,640 CHROMIUM PLATING FROM AN ORGANIC/ AQUEOUS MEDIUM James R. Brannan, Mentor, Ohio, assignor to E. 1. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Feb. 19, 1971, Ser. No. 117,102

Int. Cl. C231) 5/06 US. Cl. 20451 8 Claims ABSTRACT OF THE DISCLOSURE Deposition of bright, decorative chromium plate is obtained from a chromium plating medium that is a homogeneous liquid blend of Water with dipolar organic compound. The medium contains a complex, water-soluble trivalent chromic compound for plating that contains carboxylic acid constituents and halogen constituents and exhibits ready water solubility. The chromium deposits from such a medium exhibit enhanced plating coverage.

BACKGROUND OF THE INVENTION Decorative chromium plating from baths containing chromium in the trivalent state, wherein the baths are a blend of water and dipolar organic compound, has been shown, for example, in British Pat. 1,144,913. Such a bath, typically using a blend of water with dimethyl formamide and containing chromic chloride hexahydrate, has shown commercial promise. However, in working such baths, the most desirable performance in coverage of the chromium deposits, particularly at the low current density area, is not always achieved.

SUMMARY OF THE INVENTION It has now been found that in the plating of decorative chromium plate from media which are blends of water with dipolar organic compounds, that the coverage of the deposited chromium plate can be desirably enhanced, particularly in the low current density region.

Broadly, the invention is directed to an electrolytic plating medium, for the plating of bright chromium plate, which medium comprises a homogeneous liquid mixture of water and dipolar organic compound, the proportion by volume of water to dipolar organic compound being within the range of 3:1 to 1:3, and the medium containing a complex, water-soluble trivalent chromic compound containing carboxylic acid constituents and halogen constituents selected from the group consisting of chlorine, fluorine, bromine, iodine and mixtures thereof, such medium having a molar concentration of chromium within the range from about 0.5 to 3.

The invention is further directed to the method of chromium plating an article with a decorative chromium plate from a plating medium showing enhanced deposition in the low current density area and also directed to resulting articles thereby plated.

DESCRIPTION OF PREFERRED EMBODIMENTS The dipolar organic compound is blended with water toprovide a volume ratio of water to dipolar organic compound of between about 3 :1 to 1:3 and preferably, for best plating efiiciency e.g., enhanced conductivity and augmented plating coverage, the dipolar organic compound and water are present, by volume, in a ratio between about 3: 12 to 2: 3.

The complex, water-soluble chromic compound for plating contains carboxylic acid constituents plus halogen constituents which can be chlorine, fluorine, bromine, iodine, or mixtures thereof. However, in typical commercial plating operation, bromine and iodine are often not used, for economy and to avoid evolution of visible noxious fumes at the anode. Thus chlorine and fluorine are preferred.

Although it is not meant that the complex, water-soluble chromic compound can contain carboxylic acid constituents that are representative of only a special group of carboxylic acids, such acids which can or have been used for the chromic compounds are typically exemplified by monocarboxylic and dicarboxylic acids, which may also contain hydroxyl groups, e.g., one or two such groups. For plating efficiency and water-solubility, advantageously these acids are non-aromatic acids containing less than about 10 carbon atoms; representative acids include glycolic acid, lactic acid, and their mixtures. Preferably, for enhanced plating performance plus economy the chromic plating compound used virtually always contains carboxylic acid constituents supplied at least in major amount by glycolic acid. A compound of any of these acids such as a salt or an ester thereof, which acts in any of the reactions, such as those discussed in more detail hereinbelow whereby the complex is formed, in the same manner as the free acid, may also be used.

One method is the straightforward combination of chromium metal with carboxylic acid plus hydrochloric acid. When such combination includes particulate chromium metal to reduce reaction time, the reaction can be highly exothermic, and therefore caution need be taken in carrying out same. Typically for enhanced reaction efl'lciency, as the reaction proceeds and the evolved heat starts to diminish, external heating is applied; and, where the reaction proceeds in aqueous medium such external heating can involve refluxing of the reaction mixture to augment completion of the reaction.

The complex of this type may also be prepared from the carboylic acid and hydrochloric acid in admixture with chromic acid, typically charged to the reaction medium as a solution of chromic acid in water. The chromic acid can be supplied by any of the suitable substances for forming chromic acid in water, e.g., chromium trioxide. The reaction resulting from this method is also exothermic and caution in the use of such method is also thus advisable. These complexes may further be prepared by reaction of chromic halide, with such halide corresponding to the halide that is to be present in the complex, which chromic halide is reacted with the carboxylic acid, this reaction further involving the addition of strong base, e.g., an alkali metal hydroxide. For example,

may be used in this method and will readily yield a chromium/carboxylic acid/fluoride complex involving exothermic reaction conditions.

These carboxyl containing complexes virtually always contain a molar ratio of chromium atoms to carboxylate constituent within the range of 120.7 to 1:3.0. Where halogen is present the complex essentially always has a molar ratio of chromium atoms to halogen atoms Within the range of 120.1 to 1:35. Especially preferred ratios, based upon desirable plating performance and economy can depend upon the acid and also upon the halogen constituent when such is present. Thus for example, for a chromic carboxylate prepared with glycolic acid, the ratio of the chromic ion to glycolic is preferably maintained within the range from about 1:1.1 to 1:2.1. For a complex containing a substantial amount of the glycolic acid for the carboxylate, which complex further contains chloride as the major amount, to all, of the halogen, the ratio of chromium atoms to halogen is preferably within the range of about 1:0.4 to 1:1. However, when the halogen in such a complex is preponderantly, to all, fluoride, the ratio of chromium atoms to halogen is preferably within the range of 1:2.6 to 1:32.

The complex is generally present in the bath in an amount to provide from about 25 to about 150 grams of chromium per liter, that is, the molar concentration of chromium in the plating medium is within the range from about 0.5 to about 3.0. The more highly concentrated baths having augmented viscosity are not well suited for deposition of chromium onto a substrate immersed therein. Thus such baths having a molar concentration of chromium above about 1.5 are used for spot plating, for example, brush plating. From the foregoing methods of preparation of the complex it can be appreciated that some unreacted, carboxylic-acid-supplying material, typically the free acid or an ester thereof, may be present with the chromic plating compound. It is permissible that such excess acid be present in the electrolytic plating medium and thereby form, together, such other substances as may be present in their liquid state, a very minor amount of the total liquid of the liquid plating medium.

The bath may also contain a salt of a strong acid preferably, for economy, an alkali metal salt; these salts enhance the conductivity achieved in the electroplating operation. Most preferably, for economy, the cation of the salt is sodium, potassium or their mixtures, and the strong acid anions should be halide anions, from an acid having a dissociation constant of at least K=- for example chloride. The plating medium usually contains between about 30-150 grams per liter of such salts, basis liters of the plating medium. Such medium can also contain boric acid, or an equivalent to boric acid in aqueous solution, such as borax, boron oxide, or sodium oxyfluoborate. Such compounds may operate in the plating medium to augment the rate of deposition of the chromium and are typically used in amounts between about 10-70 grams per liter of the medium.

The pH of the plating medium should be maintained within the range from about 1 to 4 and preferably between about 2-3 to, for example, retard hydrogen evolution at the cathode during plating operation. The adjustment of pH can be typically readily achieved by the use of hydrochloric acid or an alkali metal hydroxide or carbonate, e.g., sodium hydroxide. The temperature of the medium can affect the useful plating range, such range being greater at lower temperature. Thus although plating may be carried out at a temperature as elevated as 50 C., a temperature below about 30 C. is almost always used.

' During plating, the object to be plated is made the cathode, for example immersed in the plating medium, or the cathode in a spot plating system using a portable plating device supplying the electrolyte and a positive source of electrical current, e.g., brush plating operation, or the plating medium is contained in the brush and an inert anode is used such as a carbon, graphite, platinum or platinized titanium anode. Exemplary cathode substrates for receiving the plate include metals such as steel, brass, copper, copper alloys including bronze, zinc die castings and nickel. Additionally such plating can be performed on plastic surfaces which are activated or prepared for. an electroplating operation. I

When the deposition of the chromium plate is carried out in a bath, it may be carried out in any vessel useful for chromium electroplating such as tanks lined with corrosion resistant material including glass, ceramic material, polyvinyl chloride and the like. Also, electrodeposition with such plating baths may be performed y a y com/err v 4 tional plating technique including rotating receptacle coating-apparatus immersed in a plating bath. In selection of particular bath apparatus it is preferred that the anode immersed in the bath be made of graphite, for extended plating operation in a bath containing dimethyl formamide. For such baths, chlorine may form at the anode and build up in the bath causing early curtailment of continued plating operation. Preferably this is overcome by immersing the anode in an anolyte, and for this a diaphragm compartment cell is used in the plating operation. Such an anolyte can be an aqueous solution of typically an alkali metal acetate, e.g., sodium acetate, and this solution is separated from the plating solution or catholyte by the diaphragm. In the anolyte, for example one containing sodium acetate, the sodium acetate will be present in an amount of about 75 grams per liter.

The following example shows a way in which the invention has been practiced but should not be construted as limiting the invention. Plating tests in the example are conducted in a standard Hull cell. The standard Hull cell is a trapezoidal box of non-conductive material at the opposite ends of which are positioned anode and cathode plates, as has been more particularly described in US. Pat. No. 2,149,344. For the standard Hull cell, it is possible to easily determine the effective plating range of a plating composition under varying conditions. The current density at any point on a cathode is determined according to the formula A'=C(27.748 log L) wherein A is the current density in amps per square foot (a.s.f.) at the selected point, C is the total current in amps applied to the cell, and L is the distance in inches of the selected point from the high current density end of the plate.

Example 1 Into a container there is placed 0.8 mole of chromium metal, 1.8 mole of glycolic acid of 70% strength, that is, 70% of glycolic acid and a balance of water, and 0.6

. mole of 37.3% strength hydrochloric acid which is 37.3% by weight HCl in water. The container is covered and good ventilation is provided. After the ingredients are placed together in the container, dissolution of the chromium starts slowly but gradually increases thus supplying heat to the reaction. As the reaction continues the temperature of the reaction medium reaches about 70 C. without external heating and the chromium metal can be seen by visual inspection to be substantially dissolved. As the temperature starts to subside from about 70 C., external heating is applied and the temperature of the reaction medium is permitted to reach 88 C. until all the chromium is dissolved. Total reaction time, i.e., to complete chromium metal dissolution, is about 4 hours. Thereupon the solution is heated at reflux, reaching a tempera mitted to cool.

The resulting complex, having a molar ratio of chrominum to glycolic acid of 112.25 and of chromium to chloride of 1:075 is evaporated to form an ostensibly hydrated solid. Portions of this solid are dissolved in various blends of organic liquid and water, as shown in the table below, to prepare plating solutions for testing.

The particular Hull cell used has acapacity of 267 milliliters and uses graphite anodes, with the cathode for each test being a 3 by 2%" brass panel, each panel being nickel coated prior to use in the cell. As shown in the table below, each test is carried out using one ampere current for a 3 minute cycle and at a voltage as shown in thetable below.

All panels plated well at the high current density level, about a.s.f. for a panel plated at a current of one amp, so that the deviation in the plating range percentage on the panels was determined from observation of the plating deposit at the low current density level. Plating range as a percentage is that portion across the length of the panel on which a deposit is obtained, measured from the j high current density area, divided by the total p n l l gth.

In the table below, abbreviations have been used to represent organic liquids employed with S representing sulfolane, TMU for tetramethylurea, DMF for dimethyl formamide, and DMA for dimethyl acetamide. For each experiment, the temperature of the plating solution and the pH, where such has been measured, are given in the table below along with the concentration of boric acid employed for each test. Where pH adjustments is necessary before plating, concentrated aqueous solutions of sodium hydroxide have been used.

4. The plating medium of claim 1 wherein said medium additionally contains a salt of a strong acid, the cation of said salt selected from the group consisting of sodium and potassium and their mixtures, and the anion of said salt is a halide, and a substance selected from the group consisting of boric acid, a substance supplying boric acid equivalent in water, and mixtures thereof.

5. The plating medium of claim 4 wherein said plating medium contains between about 30-150 grams per liter of said salt, basis liters of plating medium.

TABLE Plating range, Complex, Organic Vol. H3303, percent moles/l. liquid percent moles/l. pH F. Amps Volts on panel 0.5 60 0.5 N.A. 90 1 15 61 0.5 TMU 60 0.5 N.A. 80 1 23 70 1.0 DMF 60 0.6 2.5 80 1 15 66 1.0.. DMF 60 0. 5 3.0 80 1 14 62 0.6 DMF 40 0.5 3.0 85 1 8 61 1.0 DMF 43 1. 0 3.0 80 1 9 62 1.0 DMA 60 0.5 2.0 80 1 23 70 0.6---. DNA 40 0.5 3.0 80 1 11 64 1.0 DMA 40 1. 0 3. O 80 1 11 66 Comparative: 1.0 DMF 50 0.15 2 80 I 8 52 OCH.

1 Chromic chloride hexahydrate.

Norn.-N.A. =Not available.

As can be seen from the above results, and focusing on no 6. The plating medium of claim 4 wherein said subthe right hand column, plating with solutions containing the chromium glycolate complex can offer plating coverage over a more greatly extended range, from a variety of organic liquids. The comparative system has been prepared in accordance with the precepts of British Pat. No. 1,144,913 and in the above table yields a result consistent with the results disclosed in such patent for such system. In addition to the substances listed above, this comparative bath contains one mole of sodium chloride and 0.6 mole of ammonium chloride. Such comparative bath can be expected to provide a plating deposit down to about 12 a.s.f. Compared to this, the plating baths of the present invention can yield chromium deposits down to about 7 a.s.f. under the plating conditions of this example.

I claim:

1. An electrolytic plating medium for the plating of bright chromium plate, which comprises a liquid blend of water and dipolar aprotic organic compound selected from the group consisting of dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, bis(2-methoxyethyl) ether, sulfolane, tetramethyl urea and mixtures thereof forming a homogeneous mixture with water, with the volume ratio of water to dipolar organic compound being within the range of 3:1 to 1:3, and with the blend containing a complex, water-soluble trivalent chromic compound containing halogen constituents selected from the group consisting of chlorine, fluorine, mixtures thereof and mixtures with other halide, said complex also containing carboxylic acid constituents supplied in major amount by glycolic acid, said medium having a molar concentration of chromium within the range from about 0.5 to about 3 and said complex having a molar ratio of chromium atoms to carboxylic acid constituent within the range of 1:0.7 to 1:3.0 and a molar ratio of chromium atoms to halogen atoms within the range of 1:01 to 1:35.

2. The plating medium of claim 1 wherein any balance of said carboxylic acid constituent contains less than about carbon atoms and is a non-aromatic acid selected from the group consisting of dicarboxylic acids, monocarboxylic acids, monocarboxylic and dicarboxylic acids containing at least one hydroxyl group, and mixtures thereof.

3. The plating medium of claim 1 wherein said medium is maintained within a pH of between about 1-4 and at a temperature not substantially above about 50 C.

stance supplying boric acid equivalent in water is selected from the group consisting of borax, boron oxide, sodium oxyfluoborate, and mixtures thereof and said plating medium contains between about 1070 grams per liter 0 of said substance, basis liters of plating medium.

7. The method of plating an article with bright chromium plate comprising passing a current between an anode and an article forming a cathode that are in contact with a plating medium which comprises a liquid blend of water and dipolar aprotic organic compound selected from the group consisting of dimethyl formamide, dimethyl acetamide, dimethyl sulfoxide, bis(2 methoxyethyl) ether, sulfolane, tetramethyl urea and mixtures thereof forming a homogeneous mixture with water, with the volume ratio of water to dipolar organic compound being within the range of 3:1 to 1:3, and with the blend containing a complex, water-soluble trivalent chromic compound containing halogen constituents selected from the group consisting of chlorine, fluorine, mixtures thereof and mixtures with other halogen, said complex also containing carboxylic acid constituents supplied in major amount by glycolic acid, said medium having a molar concentration of chromium within the range from about 0.5 to about 3 and said complex having a molar ratio of chromium atoms to carboxylic constituent within the range of 1:0.7 to 1:30 and a molar ratio of chromium atoms to halogen atoms within the range of 1:0.1 to 123.5.

8. The method of claim 7 wherein said plating medium is maintained at a pH of between about 1-4 and at a temperature not substantially above about 50 C.

References Cited UNITED STATES PATENTS 1,922,853 8/ 1933 Kissel 2045 1 2,517,441 8/1950 Raab 20451 3,006,823 10/ 1961 Deyrup 20451 3,021,267 2/ 1962 Berzins 2045 1 FOREIGN PATENTS 697,225 9/ 1953 Great Britain 2045 1 1,144,913 3/ 1969 Great Britain 20451 136,147 6/1960 U.S.S.R. 20451 F. C. EDMUNDSON, Primary Examiner 

